The present invention relates to a method for controlling a temperature, and more particularly, to a temperature control method that is suitable for controlling a temperature of air within a chamber called "freezing container" in which articles to be transported are accommodated.
One example of a temperature control method in a freezing container in the prior art will be explained with reference to FIG. 1. In this figure, reference numeral (10) designates a chamber in which articles to be transported are accommodated, and in this chamber, air is circulated through an evaporator (4) and a heater (11), as indicated by blanked bold arrows in the figure, by driving a fan (12). Reference numeral (1) designates a refrigerant compressor, and a refrigerant gas at a high temperature and at a high pressure discharged from this refrigerant compressor (1) enters a condenser (2) in which the refrigerant is cooled and condensed into liquid; subsequently, the refrigerant enters a restrictor (3) in which it expands adiabatically, and then it enters the evaporator (4), in which the refrigerant cools the air circulating within the chamber (10), while it is in itself heated and evaporates into a gaseous state and returns to the compressor (1). During operation of the compressor (1), by opening a valve (5), a part of the refrigerant gas at a high temperature and at a high pressure discharged from the compressor (1) by-passes the condenser (2) and the restrictor (3) and enters the evaporator (4) without radiating heat, hence a cooling capacity of the evaporator (4) can be reduced, and the cooling capacity can be controlled in a stepless manner by regulating a degree of opening of the valve (5). Reference numeral (8) designates a temperature detector for detecting a temperature of a return air (6), numeral (9) designates a temperature detector for detecting a temperature of a blow-out air (7) which has been cooled or heated by passing through the evaporator (4) and the heater (11), and numeral (14) designates temperature detector selecting means, which selects the use of the temperature of the blow-out air (7) detected by the temperature detector (9) in response to a signal applied thereto from temperature setting means (15) when a set temperature arbitrarily preset in the temperature setting means (15) correspnds to a cold storage condition, but which selects the use of the temperature of the return air (6) detected by the temperature detector (8) in response to a signal applied thereto from the temperature setting means (15) when the set temperature arbitrarily preset in the temperature setting means (15) corresponds to a freezing condition. Reference numeral (16) designates a deviation calculator, which compares the detection temperature selected by the temperature detector selecting means (14) with the set temperature preset in the temperature setting means (15), and calculates a deviation therebetween, and outputs the deviation to an operation instructor (20). Reference numeral (17) designates thermostat selecting means, which selects a cold storage thermostat (18) in response to a signal applied from the temperature setting means (15) when the set temperature preset in the temperature setting means (15) corresponds to a cold storage condition, but which selects a freezing thermostat (19) in response to a signal applied from the temperature setting means (15) when the set temperature preset in the temperature setting means (15) coresponds to a freezing condition. In the operation instructor (20), the deviation input thereto from the deviation calculator (16) and a deviation preset in the thermostat (18) or (19) which is also input thereto from the thermostat selecting means (17) are compared, and the operation instructor (20) issues instructions for operating the compressor (1), the valve (5), the heater (11) and the fan (12) under predetermined operation modes according to an operation pattern preset in the thermostat (18) or (19).
In the case where frozen articles are accommodated within the chamber (10) and the temperature of the chamber is to be maintained at a predetermined temperature or lower, for instance, if -18.degree. C. is preset in the temperature setting means (15), then in response to signals issued from the temperature setting means (15), the temperature detector selecting means (14) selects the temperature selector (8) and the thermostat selecting means (17) selects the freezing thermostat (19). In the event that the detection temperature of the return air (6) detected by the temperature detector (8) is higher than the set temperature preset in the temperature setting means (15), that is, higher than -18.degree. C., then a deviation between these temperatures is calculated in the deviation calculator (16) and input to the operation instructor (20), in which the deviation is compared with the deviation preset in the freezing thermostat (19) and if the former deviation is higher than the latter deviation, the refrigerant compressor (1) and the fan (12) are started in response to instructions issued from the operation instructor (20) and hence the air circulating through the chamber (10) is cooled by the evaporator (4). If the air temperature is lowered and the deviation of the detection temperature from the set temperature becomes equal to or lower than the deviation preset in the freezing thermostat (19), then the refrigerant compressor (1) is stopped in response to an instruction issued from the operation instructor (20), and subsequently, by repeating the above-described start and stop of the refrigerant compressor (1), the temperature within the chamber (10) can be maintained at -18.degree. C. which is preset in the temperature setting means (15).
On the other hand, in the case where 0.degree. C., for instance, has been preset in the temperature setting means (15) for the purpose of accommodating cold storage articles within the chamber (10), then the temperature detector selecting means (14) selects the temperature detector (9) and the thermostat selecting means (17) selects the cold storage thermostat (18). In the event that the detection temperature of the blow-out air (7) detected by the temperature detector (9) is higher than the set temperature, that is, 0.degree. C., when the deviation between these temperatures is higher than the deviation preset in the cold storage thermostat (18), in response to instructions issued from the operation instructor (20) the compressor (1) and the fan (12) are driven to cool the air, and as the temperature within the chamber (10) falls, when the deviation of the detection temperature from the set temperature has become equal to or lower than the deviation preset in the cold storage thermostat (18), in response to an instruction issued from the operation instructor (20) the valve (5) is opened to reduce the cooling capacity of the evaporator (4). Thereafter, while the operation of the compressor (1) is continued, the cooling capacity of the evaporator (4) is regulated by regulating a degree of opening of the valve (5) in response to an instruction issued from the operation instructor (20). However, in the event that the thermal load for the chamber (10) has been lowered in excess of a controllable range by means of the valve (5), then a current is passed through the heater (11) in response to an instruction issued from the operation instructor (20), and thereby a balance between the thermal load for the chamber (10) and the cooling capacity of the evaporator (4) can be maintained. In the case where the deviation of the detection temperature from the set temperature is equal to or lower than the deviation preset in the cold storage thermostat (18), the heater (11) and the fan (12) are started without starting the compressor (1), and the circulating air is thereby heated, and at the time point when the temperature within the chamber (10) has risen and the deviation of the detection temperature from the set temperature has reached the deviation preset in the cold storage thermostat (18), the compressor (1) is started.
In the above-described control system in the prior art, the operation of the compressor (1) is always continued during cold storage, and the cooling capacity is decreased or increased in a stepless manner by increasing or decreasing a degree of opening of the valve (5) in a stepless manner, and a high precision temperature control is thereby effected so that the variation range of the temperature of the air within the chamber (10) may be reduced to as small a value as possible. However, in the case of cold storage of industrial products such as films, electric products, etc., the set temperature to be preset in the temperature setting means (15) is in a high temperature region in the proximity of 20.degree. C., and also in this case, a temperature control at a high precision having a very narrow variation range of the temperature within the chamber as is the case where agricultural products such as fruits, chilled beef, etc. are stored under a cold condition at a set temperature in the proximity of 0.degree. C., is unnecessary; and furthermore, since the set temperature is in a high temperature region in the proximity of 20.degree. C., the thermal load is low and hence a period when cooling of the air in the chamber is unnecessary would arise frequently. Therefore, it is wasteful to carry out high precision temperature control while the operation of the compressor (1) is always continued despite of the above-mentioned facts; the power necessitated for the operation of the compressor (1) during this temperature control is wasted, and so, the temperature control system in the prior art had a shortcoming in that its efficiency of energy consumption was very poor.